Flatbed scanning or copying devices, or “multi-function peripherals” (MFPs) typically provide a glass plate for supporting a document to be scanned or copied by the device. Light for illuminating the document is transmitted through the glass plate to the document, and the light is reflected by the document back through the glass plate to light sensitive components in the device to produce an image of the document. Therefore, any light blocking or light scattering defects on the glass plate will leave an “artifact” on the image, distorting or corrupting the image.
Such defects are either transitory or non-transitory. Transitory defects, such as dust, hair, and chemical deposits, are generally removable, though it is recognized herein that some of these otherwise transitory defects may not be accessible for removal by a user of the device because the deposits have become lodged on the underside of the platen glass. This is a not uncommon occurrence in low-end, consumer type scanners which may not be sealed well against such incursions. Non-transitory defects, such as scratches and pits in the glass, are not removable. Both types of defects are common, particularly in relatively inexpensive consumer grade scanning devices.
Defects can be accounted for and the image can be corrected using known interpolation techniques. This accounting can be done initially at the factory and at any time by the user during use of the apparatus, and is preferably done throughout the life of the apparatus because defects will typically accrue through use. In a prior art methodology, the platen glass is wiped clean, and it is assumed that any defects remaining are permanent. The apparatus is then used to image the platen without a document, so that all of the reflected light corresponds to the remaining defects. An interpolation algorithm is used to interpolate between pixels corresponding to locations on the platen glass for which no reflected light is received that span one or more pixels for which reflected light is received. However, a number of the defects remaining after a cleaning may actually be transitory defects that have merely been moved from one location on the platen glass to another. Similarly, cleaning can often add defects to the platen glass, by transferring defects existing on or in a cleaning device, such as dust or lint on or in a cloth.
It is recognized herein that it is undesirable to correct the image for defects that are transitory because these may either be removed or moved during use (hereinafter “removable”). Further, correction imposes a cost in image quality, as some information is typically lost in interpolating, so that correction will introduce distortion if it is not needed, or if it is applied at an inappropriate location.
Accordingly, there is a need for a method and apparatus for mapping defects on the light-transmissive support surface of a document scanning or photocopying device that provides for distinguishing defects for which image correction should be performed as a result of the defects being permanent, and defects for which image correction should not be performed as a result of the defects being removable even though they may not actually have been removed.